How to check Memory Usage on Linux?

It shows how much your applications are using system DRAM. It is roughly lower than Physical Memory. Linux Systems cache some disk data. Caching is the difference between Physical and Real Memory. The cached disk space will be made available to applications though on demand.

Swap Memory

Swap is additional space to your actual DRAM. This space is borrowed from disk space and once you application fill-out entire DRAM.

From Mem section (first line) above: shows memory usage including buffers/cache etc. Therefore free memory is shown as less. But remember, buffers/cache is temporary storage, a performance improvement trick of Linux. That memory is actually free and made available on demand.

We must look at the second line: -/+ buffers/cache section: used & free values – to find memory situation.

In this case,

15509664K (15GB) Memory is free (This is what is really free memory that is available to you).

814976K (814 MB) is used.

So, 15GB is actually a sum of free + buffers + cache from first line (7010520 + 433096 + 8066048).

Resulting data is in KB by default. Use free -m switch to see numbers in MB. free -sm 5 will refresh data every 5 seconds. watch free will do same without scrolling.